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I'm on Linux, and have installed nghttp2.

If you're running a version of Wireshark that your distribution provides as a package, then:

If it was built with nghttp2, installing it should have installed nghttp2 (most Linux packaging systems keep track of dependencies and install required packages). In that case, installing nghttp2 shouldn't make a difference.

If it wasn't built with nghttp2, installing nghttp2 shouldn't make a difference.

If you're running a version of Wireshark that you built yourself, you would have to have installed nghttp2, including a developer package for nghttp2, before configuring the build and building Wireshark.

Does the "About" dialog for Wireshark say it's built with nghttp2? It would be mentioned in the "Compiled" section.